1. FIELD OF THE INVENTION
The present invention relates to a semiconductor laser device More particularly, it relates to an improved structure of a semiconductor laser device capable of starting a transverse fundamental mode by a low threshold current and making its manufacture feasible.
2. DISCUSSION OF BACKGROUND
FIG. 2 is a diagram showing a conventional planer stripe type laser device formed by using an impurity diffusion method as a semiconductor laser device of this kind.
The conventional planer stripe type semiconductor laser device having the construction as in FIG. 2 is fablicated as follows. On an n-GaAs substrate 11, an n-AlGaAs layer 12, a GaAs layer 13, a p-AlGaAs layer 14 and an n-GaAs layer 15 are respectively formed by using a growth technique; by selectively diffusing Zn in a stripe form having the width about 10 .mu.m from the uppermost n-GaAs layer 15 until Zn reach the p-AlGaAs layer 14 to form a Zn diffused p-region 16 indicated by hatched lines to thereby reversing the n-GaAs layer 15 into a p-type conducting layer, and by forming a p-side electrode 17 on the n-GaAs layer 15 while an n-side electrode 18 is formed on the n-GaAs substrate 11.
In the conventional semiconductor laser device, when a forward biasing voltage is applied to it, the region of the p-AlGaAs layer 14 and the n-GaAs layer 15 except for the Zn diffused p-region 16 is reversely biased, whereby a current is concentrated to the Zn diffused p-region 16. As a result, the density of injection of carriers becomes large at a portion in the GaAs layer 13 just below the Zn diffused p-region 16 and there takes place a laser beam and at the same time, a high gain is obtainable at that portion just below the Zn diffused p-region 16 with the result that amplification of lightwaves occurs due to stimulated emission; thus, the portion just below the Zn diffused p-region 16 constitutes a waveguide region.
As described above, the conventional planer stripe type semiconductor laser device is so constructed that a current is restricted by a p-n reverse bias junction. In this case, it is difficult to reduce a threshold current since the current spreads from the Zn diffused p-region 16 to the p-AlGaAs layer 14. In addition, horizontal transverse mode of emission is apt to be unstable because the conventional semiconductor laser device is of a gain waveguide type wherein a refractive index waveguide path is not formed.